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Description
Hydrogen damage in the shape of blistering was noticed on the first shell course of a low-carbon steel atmospheric storage tank for petroleum products. The blisters were identified both on the internal and external surfaces of the plates, whereas no blistering was observed on the remaining shell courses. To investigate the influence of the blistering on the structural integrity of the tank shell, samples were taken out from areas of the plates affected with blisters and without any visible blistering. Tensile and bend tests were performed on the samples, as well as metallography and chemical composition analysis.
The investigation showed reduced mechanical properties, with both the ultimate tensile strength (Rm) and elongation (A) being lower than the standard defined values for low-carbon steel grade VSt3ps6. The bend test yielded visible cracks on the specimen during a bending angle <90o. The chemical composition analysis showed slight differences in the blistered and non-blistered samples, most notably higher content of S and P on the blistered surface. Comparing the obtained values with the chemical composition of the steel grade VSt3ps6 revealed that while the analyzed samples were mostly within the standard specifications, the S and Si content in the blistered area was slightly above the defined limits, while the Mn content was on the upper limit. These findings are in line with the conditions required for hydrogen damage, as both MnS inclusions and segregation of P along the grain boundaries present hydrogen trapping sites. The metallography revealed crack initiation sites on the edges of the blister, consistent with hydrogen-induced blistering, as well as visible presence of non-metallic inclusions.
The results revealed significant degradation caused by hydrogen, manifested by deterioration of the mechanical properties and alterations in the chemical composition of the steel. The research demonstrated that the combined application of mechanical testing, chemical composition analysis, and metallographic examination constitutes an effective approach for the detection and characterization of hydrogen damage in low-carbon steel atmospheric storage tanks. Furthermore, the findings provide valuable insight into the mechanisms governing hydrogen blister formation and establish a basis for future investigations aimed at determining the root causes of hydrogen-induced blistering.
Keywords: hydrogen blistering, low-carbon steel, mechanical properties